1. Field of the Invention
The invention relates to the field of DC power supplies.
2. Prior Art
DC power supplies which employ transformers to convert a first DC potential to a second DC potential are well-known. In some cases, these supplies are single-ended, that is, magnetic flux is induced in only one direction within the transformer. Most often these power supplies are of the "flyback" type, that is, power is transferred after conduction has ceased in the primary winding. Regulation of the output voltage is accomplished by controlling the amount of energy stored in the magnetic field of the transformer. This is generally done by controlling the peak current in the primary winding through a power transistor.
For the self-exciting type of power supply, an auxiliary drive or oscillator is often employed to initiate oscillations. As will be seen, the starting/restarting circuitry employed with the present invention consists substantially of passive circuit components which are inherently reliable. This circuitry also provides excellent fault protection.
In flyback systems employing transformers, a problem has existed in dissipating all the energy stored in the magnetic field of the transformer. Because of stray inductance and capacitance, and because complete magnetic coupling does not occur between the primary and secondary windings, all the energy introduced in the magnetic field from the primary winding is not coupled to the secondary winding. Typically, some of the energy stored in the field is dissipated in the power transistor which controls the current in the primary drive winding and in damping diode networks, etc. This subjects these components to severe stress in some cases. Moreover, when a fault condition occurs such as an open secondary, or a no-load condition, all of the energy stored in the magnetic field must be dissipated on the primary side. As will be seen with the present invention, an additional primary winding is employed which in effect returns a substantial amount of the energy in the magnetic field which is not transferred to the secondary winding, back into the primary power supply.